There is a need for a simple, low cost and energy efficient dimming circuit for a single conventional double end fluorescent lamp when employed to illuminate the work areas of modern office furniture such as desks or drawing boards. Presently available dimming circuits therefor are relatively expensive and in most cases the power imput does not decrease in proportion to the decrease in light level. I have found that a commercially available, rapid start type two lamp ballast sells for the same price as a single lamp ballast of the same type and provides the two independent secondary windings necessary to maintain a constant filament voltage at both ends of a single fluorescent lamp to insure conduction when current through the lamp is substantially reduced by impedence for the purpose of dimming light output.
I have also found that objectionable arcing and noise incidental to switching an uncharged capacitor in parallel with a charged capacitor may be dramatically minimized by placing a low ohmic resistor in series with the charged capacitor.
In a first form of the invention wherein economical employment of capacitors to effect dimming is a feature, two capacitors of different value are progressively connected in series with the lamp and ballast by a rotary switch, first in parallel with each other, second and third individually and fourth in series with each other so as to progressively reduce current flow through the lamp thereby to progressively dim the lamp in four steps.
In a second form of the invention a first capacitor is connected in series with the lamp and ballast to further limit the maximum light output of the lamp in accordance with the relative positions of the lamp and a work area and in accordance with a calibrated task light lens so as to provide a desirable maximum illumination of the work area. Additionally four capacitors having progressively less capacitance are selectively connected in parallel with the first capacitor by a four position slide switch to selectively establish with the first capacitor a maximum light output and to further reduce or dim the light output below this maximum. A resistor is connected in series with the first capacitor to substantially eliminate arcing and noise incidental to the connection of any of the four uncharged capacitors with the first charged capacitor. In both first and second forms of the invention the ballast and starting circuit includes a step up auto transformer and in both forms sufficient voltage across the lamp filaments at the ends of the lamp is supplied by separate secondary windings of the transformer to insure maintenance of conduction when current flow through the lamp is substantially reduced to effect dimming.